This application requests support for studies pertaining to the principal investigator's recent finding that glucocorticoids (e.g., dexamethasone) trigger apoptosis in neural progenitor cells (NPCs) in the external granule layer of the cerebellum. Glucocorticoids induce the death of cerebellar NPCs only during a brief period when these cells are dividing and subdividing in order to produce neurons that will subsequently become granule cells in the internal granule layer and, possibly, interneurons in the molecular layer. In rodents this period occurs between postnatal days 4-10. In humans the corresponding period would last from the 20th week to the 45th week post conception. After cerebellar neurogenesis has been accomplished, the NPCs are removed by apoptosis. The signal that these NPCs use to decide when to begin to kill themselves by apoptosis is unknown. Determining what the signal might be has important implications for developmental neurobiology. The fact that 11[unreadable]-hydroxysteroid dehydrogenase type 2, an enzyme that breaks down only endogenous glucocorticoids, disappears from the external granule layer of the cerebellum at the end of neurogenesis period, when NPCs undergo apoptosis, suggests that endogenous glucocorticoids might be the natural signal. Clinically, exogenous glucocorticoids (e.g., dexamethasone, betamethasone) are given to mothers, who are at high risk of giving birth prior to 32 weeks gestation, in order to induce maturation of the fetal lungs. In addition postnatal premature infants receive corticosteroids for 2-42 days either to prevent or treat chronic lung disease. Follow-up data from clinical trials suggest that infants exposed to glucocorticoids in addition to having developmental delays and smaller brains and bodies, have impairments in motor skills, motor coordination, and visualmotor integration. These data have raised concern about the relative safety of glucocorticoid exposure in the human infants. The histological work proposed in Specific Aim 1 of this application seeks to determine that glucocorticoids produce apoptosis by activating glucocorticoid receptors and that endogenous glucocorticoids are less likely to produce this toxicity than synthetic ones. Then the long-term effect of a single dose of glucocorticoids on cerebellar neuronal numbers and on motor and coordination tasks will be studied in Specific Aim 2. Results of the proposed studies could provide immediate guidance on which glucocorticoids might be less toxic in humans. In addition the findings will serve as a basis for conducting future studies aimed at more fully understanding the basic biology underlying decisions about when cells undergo apoptosis, and on finding treatments that might be more effective in preventing this drug-induced apoptosis. [unreadable] [unreadable] [unreadable]